A parking lot management system is an example where a system that manages the number of objects present in a predetermined place is applied. In a parking lot management system, the number of vehicles that can enter the parking lot is managed by counting the number of vehicles entering through an entrance gate and the number of vehicles leaving through an exit gate. A control is provided so that vehicles of a number larger than the number of vehicles that can be park in the parking lot do not enter the parking lot. In this system, however, although the total number of vehicles in the parking lot can be managed, it cannot be ascertained which specific place is empty.
To ascertain the condition of parked vehicles, for example, in an indoor parking lot, a method of installing an infrared sensor on the ceiling is known. In this method, however, because a plurality of vehicles cannot be detected with one infrared sensor, due to the physical directional characteristics of infrared rays, many infrared sensors need to be installed, thereby increasing cost. If only a few sensors are installed to reduce the cost, all the parking places cannot be covered.
A conventional parking management system for solving these problems has been proposed (for example, see Patent Document 1). In this parking management system, a vehicle detector that detects presence or absence of vehicles in a parking area is movably fitted on a rail provided so as to span over a plurality of parking areas. Further, a barcode including position information of the parking area is attached to a predetermined position of the rail, so that the vehicle detector can detect the presence or absence of vehicles at predetermined positions of the parking area. In other words, when a barcode reader included in the vehicle detector reads the barcode, the vehicle detector stops and detects whether a vehicle is present or not at a position corresponding to the position of the vehicle sensor. The result is transmitted to a host computer connected to the vehicle detector, together with the position information of the parking area where the detection is performed.
Patent Document 1: Japanese Patent Application Laid-open No. H10-64000.